Border tunnels are of three general types: Large and Deep, Small and Shallow, and lastly, those Connecting to Existing Infrastructure.
Border tunnels are constructed because it is often far more efficient to spend weeks or months building a tunnel to transport some amount of contraband than to risk sending that same amount contraband on the surface where it might easily be detected.
It is often a simple price-performance tradeoff. If the smuggler has a million pounds of drugs to move — or a million pounds of illegal aliens from Afghanistan — then a tunnel can be a low cost solution. If the contraband to be smuggled is of tremendous value — for example a key terrorist or a nuclear weapon — then a tunnel is probably the best choice.
There are all sorts of ways of finding tunnels. Some methods work and some do not. The methods of tunnel detection in use along the U.S. / Mexico border seem to come and go in waves, like fads or the lengths of miniskirts. A major problem for the USBP is that effective tunnel detection methods are deemed too imprecise and expensive.
Before one decides to build a tunnel it is imperative that the threat of possible detection be evaluated. There are four general methods of detecting tunnels:
Seismic activity (low frequency digging noises and tunnel transit noises)
Magnetic Anomaly (electrical power lines in the tunnel, metal digging tools or metal shoring)
Acoustic Activity (higher frequency noises)
Density Anomalies (the tunnel is a hole and so the amount of earth below the surface changes when you dig a tunnel)
Present urges within the Department of Homeland Security are for immediate gratification. They have this Giant Urge to just walk out along the border and — while using some magical tool — find a tunnel in ten minutes. This does not happen.
Although this does not happen, it is this thing that does not happen that DHS expects to happen and so they continue to use various sensor systems over and over again expecting a different result.
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